Formula: Fe2Si2O5(OH)4.2H2O Monoclinic
Description: Hisingerite is common as an alteration product in many iron rich rocks. Although reported in only one locality in the state, it is likely present in a number of other places. Its earthy character, yellow brown streak and association with iron oxides of similar appearance makes it easy to overlook.
MARATHON COUNTY: “Earthy masses of reddish brown to reddish ochre color up to 6 cm. in greatest diameter” are found in the vugs of pegmatite dikes of the Wausau pluton. Many of these are exposed temporarily in the “rotten granite” working south of Rib Mountain in sec. 19 and 20 T.28N. R.7E. (Falster, 1987; Falster et al., 2000).